1. Field
The present invention relates generally to position determination, and more specifically to a method and apparatus for performing position determination in a wireless communication network (e.g., a cellular network) with repeaters.
2. Background
A common technique to determine the position of a terminal is to ascertain the amount of time required for signals transmitted from multiple transmitters at known locations to reach the terminal. The propagation times for the signals are typically converted to “pseudo-ranges”, which are estimates of the distances between the terminal and the transmitters. The position of the terminal may then be estimated based on the pseudo-ranges to the transmitters and their locations, using a process often referred to as “trilateration”.
One system that provides signals from a plurality of transmitters (satellites) at known locations is the well-known Global Positioning System (GPS). An accurate three-dimensional position estimate (or “fix”) for a terminal may be obtained based on signals received by the terminal from a sufficient number of GPS satellites (typically four). However, the required number of GPS satellites may not be available in certain operating environments (e.g., indoors) to derive this fix. Another system that provides signals from a plurality of transmitters (base stations) at known earth-bound locations is a wireless (e.g., cellular) communication network. A two-dimensional (2-D) position estimate for a terminal may be obtained based on signals received by the terminal from a sufficient number of base stations (typically three or more).
Many cellular networks employ repeaters to provide coverage for designated areas within the network or to extend the coverage of the network. For example, a repeater may be used to cover a geographic area that is not covered by a base station due to fading conditions (i.e., a “hole” within the network). Repeaters may also be used to extend coverage into rural areas (e.g., along a freeway) that are outside the coverage area of the base stations. A repeater receives, conditions, and retransmits signals on both the forward and reverse links. The forward link refers to the communication link from the base stations to the terminals, and the reverse link refers to the communication link from the terminals to the base stations.
Various challenges are encountered in determining the position of a terminal in a network that employs repeaters. On the forward link, each repeater transmits a repeated signal at high power and with additional delays to the terminals within its coverage area. A terminal located within the coverage area of a repeater is often prevented from receiving signals from base stations due to a combination of the repeated signal's high power plus the isolation normally associated with the repeater's coverage area. Moreover, in many instances in which repeaters are used (e.g., inside buildings, tunnels, subways, and so on), signals from GPS satellites have insufficient power levels and also cannot be received by the terminal. Thus, a limited number of signals (possibly only one signal, from a repeater) may be available for use to determine the terminal's position.
Furthermore, the additional delays introduced by repeaters can distort measurements made by a terminal for signals received from the repeaters. Consequently, measurements for signals received from repeaters are normally discarded and not used for position determination. In certain situations, only few measurements may be available to compute the position estimate for the terminal. If signals from repeaters are discarded, then the accuracy of the position estimate obtained based on remaining signals may be very poor.
There is therefore a need in the art for a method and apparatus to provide a position estimate for a terminal in a wireless communication network that employs repeaters (or other transmission sources with similar characteristics).